Add a renderer selection option for macOS

There are numerous conflicting reports about which Mac renderer works best (even on the same systems!). I give up, just let the users figure it out.

app/streaming/session.h

@@ -184,6 +184,7 @@ private:
 
     static
     bool chooseDecoder(StreamingPreferences::VideoDecoderSelection vds,
+                       StreamingPreferences::RendererSelection renderer,
                        SDL_Window* window, int videoFormat, int width, int height,
                        int frameRate, bool enableVsync, bool enableFramePacing,
                        bool testOnly,